1. Technical Field of the Invention
The present invention relates to an orientation-following display apparatus, an orientation-following display method, and an orientation-following display program, and more particularly to an orientation-following display technology that is used for an electronic compass, a Personal Navigation Device (PND), or the like.
2. Description of the Related Art
An orientation-following display technology has been proposed, in which an image such as a map is rotated according to the posture of a screen of an electronic compass, a PND, a mobile phone, or the like so that an orientation (or direction) represented by the image follows the orientation in the real-world space. If information such as a map is displayed so as to follow the orientation, it is easy to recognize information represented by the image in association with the real-world space. In orientation-following display which is also called “heading up display”, for example, an image displayed on a screen is rotated according to the tendencies of human to recognize a front-to-rear orientation of the screen represented by the image in association with the front-to-rear orientation in the real-world space when the screen is in a horizontal state. Patent References 1 and 2 describe an orientation-following display method in which an orientation on a map corresponding to the travel direction of a human who is walking is allowed to coincide with an orientation from the bottom side to the top side or from the front side to the rear side of the screen. In the conventional orientation-following display method, an image displayed on the screen is rotated according to the magnitude of the angle between the north of the real-world space and the projection of a coordinate axis fixed with respect to the screen onto a horizontal plane of the real-world space.
[Patent Reference 1] Japanese Patent Application Publication No. 2001-289646
[Patent Reference 2] Japanese Patent Application Publication No. 2005-242283
However, in the conventional orientation-following display method, an orientation on the map corresponding to the travel direction does not follow the travel direction in the real world when the screen is at a specific posture since, in the method, one of the three orthogonal coordinate axes fixed to the screen is selected according to the posture of the screen and the image displayed on the screen is rotated according to the angle between the projection of the selected axis and the north of the real-world space. An example of switching from a state in which the image of a compass needle is rotated according to the angle between the north and the projection of a y axis, which extends in a direction parallel to a long side of the screen, onto the horizontal plane to a state in which the compass needle image is rotated according to the angle between the north and the projection of a z axis, which extends in a direction perpendicular to the screen, onto the horizontal plane is described in detail as follows with reference to FIG. 9. Here, it is assumed that the switching from the y axis to the z axis is performed at a position of y1 at which the y axis is inclined by 45 degrees with respect to the horizontal plane when a screen F is rotated about the z axis while maintaining the z axis in a horizontal state. In an interval in which the y axis is rotated from y0 to y1, a compass needle D displayed on the screen F is not rotated since the angle between the north (N) direction and the projection of the y axis onto the horizontal plane is constant. Accordingly, the disagreement between a north direction indicated by the compass needle D and the north in the real-world space gradually increases from 0 to 45 degrees in the interval in which the y axis is rotated from y0 to y1. This disagreement is subjectively recognized by the user and corresponds to the disagreement between the north of the real-world space and a north direction indicated by the compass needle D as a display target when the screen F has been rotated about an axis parallel to the lateral direction of the screen F until the screen F is parallel to the front-to-rear direction. In an interval in which the y axis is rotated from y1 to y2, the disagreement between the north direction indicated by the compass needle D and the north direction in the real-world space gradually decreases from 45 to 0 degrees. Although the image of the compass needle D has been described as a display target, such disagreements also occur when the display target is a map. In the case where an orientation represented by an image displayed on the screen does not follow the orientation in the real-world space, the user feels uncomfortable so that the confidence of the user in the displayed information is reduced or the user is confused.